The present invention relates to a DC-DC converter for supplying DC power to a load, and more specifically to a switching regulator for supplying regulated voltage to a load.
A typical switching regulator as disclosed in Japanese Laid Open Utility Model Application No. 58-25581 for example, comprises a primary winding of a transformer connected to a DC power source, a switching transistor connected in series to the primary winding, a secondary winding of the transformer, a rectifying and smoothing circuit connected between the secondary winding and a load, an error amplifier for comparing output voltage of the rectifying and smoothing circuit with reference voltage, a triangular wave generator, a comparator for comparing the triangular wave with the error output and forming PWM (pulse width modulation) signal, and a drive circuit connected to the comparator and the switching transistor. The DC-DC converter of PWM type is advantageous in that the switching transistor can be turned on or off stably at constant frequency, whereas it is disadvantageous in that since the triangular generator, the comparator and the drive circuit are required the cost becomes high.
Another typical switching regulator as disclosed in Japanese Laid Open Patent Application No. 59-148563 for example, comprises a primary winding of a transformer connected to a DC power source, a switching transistor connected in series to the primary winding, a secondary winding of the transformer, a rectifying and smoothing circuit connected between the secondary winding and a load, a tertiary winding connected between base and emitter of the switching transistor and coupled in electromagnetic coupling with the primary winding so as to drive the switching transistor, and a voltage control means. The switching transistor of the DC-DC converter having the tertiary winding is driven by positive feedback voltage obtained at the tertiary winding. Consequently, the circuit constitution of the DC-DC converter is simplified. In the DC-DC converter of positive feedback type, assuming that collector current of the switching transistor is I.sub.c, base current is I.sub.B and current amplification factor is h.sub.FE, when the collector current I.sub.c increases gradually and becomes I.sub.c =I.sub.B .times.h.sub.FE, the switching transistor is turned off. The output voltage is adjusted by varying amount of the base current I.sub.B. For example, if the base current is decreased, the ON time width of the switching transistor is narrowed and the output voltage is lowered. The base current is adjusted by bypassing a part of the base current supplied from the tertiary winding to the switching transistor. Consequently, the bypassed current produces power loss. Further in the DC-DC converter of positive feedback type, when required power of the load is small (at light load), not only the ON time width of the transistor but also the OFF time width are narrowed thereby the ON/OFF repetition frequency of the switching transistor becomes high. As a result, the number of turning-on and turning-off per unit time of the switching transistor is increased thereby ratio of the power loss based on the switching to the total power loss is increased.
Japanese Laid Open Patent Application No. 49-73615 discloses a DC-DC converter which comprises a primary winding of a transformer connected to a DC power source, a switching transistor connected in series to the primary winding, a secondary winding of the transformer, a rectifying and smoothing circuit connected between the secondary winding and a load, a tertiary winding connected between base and emitter of the switching transistor and coupled in electromagnetic coupling with the primary winding so as to drive the switching transistor, a quartic winding coupled in electromagnetic coupling with the primary winding so as to control magnetic flux of the transformer, a switch for shortcircuiting quartic winding selectively. In the DC-DC converter having the quartic winding, if the time width for shortcircuiting the quartic winding is varied, time until the magnetic flux is returned to zero, i.e., reset time is varied. As a result, the OFF time width of the switching transistor is varied. In the DC-DC converter, however, the quartic winding and the shortcircuit control circuit therefor are required thereby reduction of the cost becomes difficult.